Datasheet

LTC3677-3
21
36773f
operaTion
Ideal Diode from BAT to V
OUT
The LTC3677-3 has an internal ideal diode as well as a
controller for an optional external ideal diode. Both the
internal and the external ideal diodes respond quickly
whenever V
OUT
drops below BAT. If the load increases
beyond the input current limit, additional current will be
pulled from the battery via the ideal diodes. Furthermore, if
power to V
BUS
(USB) or V
OUT
(external wall power or high
voltage regulator) is removed, then all of the application
power will be provided by the battery via the ideal diodes.
The ideal diodes are fast enough to keep V
OUT
from drop-
ping significantly below V
BAT
with just the recommended
output capacitor (see Figure 2). The ideal diode consists
of a precision amplifier that enables an on-chip P-channel
MOSFET whenever the voltage at V
OUT
is approximately
15mV (V
FWD
) below the voltage at BAT. The resistance of
the internal ideal diode is approximately 200mΩ. If this is
sufficient for the application, then no external components
are necessary. However, if lower resistance is needed, an
external P-channel MOSFET can be added from BAT to
V
OUT
. The IDGATE pin of the LTC3677-3 drives the gate of
the external P-channel MOSFET for automatic ideal diode
control. The source of the MOSFET should be connected to
V
OUT
and the drain should be connected to BAT. Capable of
driving a 1nF load, the IDGATE pin can control an external
P-channel MOSFET having extremely low on-resistance.
Using the WALL Pin to Detect the Presence of an
External Power Source
The WALL input pin can be used to identify the presence
of an external power source (particularly one that is not
subject to a fixed current limit like the USB V
BUS
input).
Typically, such a power supply would be a 5V wall adapter
output or the low voltage output of a high voltage buck
regulator. When the wall adapter output (or buck regulator
output) is connected directly to the WALL pin, and the volt-
age exceeds the WALL pin threshold, the USB power path
(from V
BUS
to V
OUT
) will be disconnected. Furthermore,
the ACPR pin will be pulled low. In order for the presence
of an external power supply to be acknowledged, both of
the following conditions must be satisfied:
1. The WALL pin voltage must exceed approximately
4.3V.
2. The WALL pin voltage must be greater than 75mV above
the BAT pin voltage.
The input power path (between V
BUS
and V
OUT
) is re-
enabled and the ACPR pin is pulled high when either of
the following conditions is met:
1. The WALL pin voltage falls to within 25mV of the BAT
pin voltage.
2. The WALL pin voltage falls below 3.2V.
Each of these thresholds is suitably filtered in time to
prevent transient glitches on the WALL pin from falsely
triggering an event.
Suspend Mode
When I
LIM0
is pulled high and I
LIM1
is pulled low the
LTC3677-3 enters suspend mode to comply with the
USB specification. In this mode, the power path between
V
BUS
and V
OUT
is put in a high impedance state to reduce
the V
BUS
input current to 50μA. If no other power source
is available to drive WALL and V
OUT
, the system load
connected to V
OUT
is supplied through the ideal diodes
connected to BAT.
4.0V
3.8VV
OUT
3.6V
500mA
–500mA
0I
BAT
I
VOUT
LOAD
1A
0A
10µs/DIV
CHARGE
DISCHARGE
36773 F02
V
BAT
= 3.8V
V
BUS
= 5V
5x MODE
C
OUT
= 10µF
Figure 2. Ideal Diode Transient Response